Process for mounting a workpiece for machining

ABSTRACT

A process for producing a removable workpiece mounting for holding the workpiece during machining operations comprising the steps of applying a settable coating to a portion of the workpiece to provide a key, and casting a block of material around said coating when set, said cast material being solid at room temperature and said coating remaining stable at least up to the melting temperature of said cast material, said coating comprising a grit in a binder and said binder being soluble in a solvent which is non-corrosive to the workpiece material.

United States Patent Witherspoon [451 June 27, 1972 4] PROCESS FOR MOUNTING A [56] References Cited WORKPIECE FOR MACHINING UNITED STATES PATENTS [721 Ealmg England 3,191,252 6/1965 Webbere ..l64/75 [73] Assignee: Rolls-Royce Limited 3,541,674 11/1970 Sabel 164/75 2,606,482 8/1952 Sorensen et a1 ..269/7 [22] Filed: Nov. 17, 1969 g 21 A L N 882 411 Primary Examiner-J. Spencer Overholser 1 pp 0 Assistant Examiner-John E. Roethel Attorney-Mawhinney & Mawhinney [30] Foreign Application Priority Data Nov. 15, 1968 Great Britain ..54,188/68 [57] ABSTRACT A process for producing a removable workpiece mounting for [52] 0.8. CI .....29/424, 29/559, 164/75, holding the workpiece during machining operations compris- 164/1 11, 269/7 ing the steps of applying a settable coating to a portion of the [51] Int. Cl ..B23p 17/02 workpiece to provide a key, and casting a block of material [58] Field of Search 164/75, 1 1 1, 91, 98, 100, 101, around said coating when set, said cast material being solid at 164/102, 72; 249/113, 115; 269/7; 51/277; 29/527.6, DIG. 5, DIG. 8, 423, 424, 557, 559

room temperature and said coating remaining stable at least up to the melting temperature of said cast material, said coating comprising a grit in a binder and said binder being soluble m a solvent which is non-corrosive to the workpiece material.

7 Claims, 1 Drawing Figure rIIIII/IIIII.

r a I a r I I v v 4 v r 4 v v a a I d r I I g g I I p I. a I a a i I PROCESS FOR MOUNTING A WORKPIECE FOR MACHINING The present invention relates to the production of a mounting for a workpiece which will facilitate holding the workpiece while machining operations are performed on it.

According to the invention there is provided a process for producing a workpiece mounting for holding the workpiece during machining operations, the process comprising the steps of applying a settable coating to a portion of the workpiece, and casting a block of material around said coating when set, said cast-material being solid at room temperature and said coating remaining stable at least up to the melting temperature of said cast material, said coating comprising a grit in a binder and said binder being soluble in a solvent which is noncorrosive to the workpiece material.

In a preferred form of the invention the encapsulating material is a low melting point alloy e.g., an alloy sold under the trade name of CERROTRUE.

The coating may be a mixture of a cement binder, such as methyl cellulose in a solvent, and a suitable grit, a preferred mixture comprises the cement sold under the trade name of NICRO-BRAZE together with sand.

The particle size of the grit is preferably in the range between 100 and 220 British Standard mesh and in particular particle sizes of 180 British Standard mesh were found to be most suitable.

The mixture is preferably applied to the surface of the workpiece in a viscous form and caused or allowed to harden before application of the encapsulating material.

In a preferred embodiment the workpiece is an aerofoil shaped blade for a gas turbine engine and the aerofoil portion is encapsulated in a material so that it can be clamped without damage to the surface of the aerofoil portion while machining operations are performed on the blade.

An example of the invention will now be described with reference to the accompanying drawing which shows a longitudinal section of a rotor blade with the aerofoil portion coated with a mixture of cement and grit and encapsulated in an alloy matrix.

In the drawing the rotor blade 1 comprises an aerofoil 2 and an un-machined root block 3.

The aerofoil 2 has previously been completely machined to its final dimensions and surface finish. Its surface is first degreased and is provided with a coating 4 of cement binder and grit which provides a rough surface for the encapsulating material which is formed into a block 5 surrounding the coating 4. The coating 4 is a mixture of a cement sold under the trade name of NICRO-BRAZE together with grit of a particle size of 180 British Standard mesh. The grit is preferably sand but any hard grit-like material of particle size in the range 100-220 British Standard mesh could be used. The essential property of the grit is that it should provide a keyed surface to the aerofoil to which the encapsulating material can adhere and which will not break up under clamping loads. The coating must be readily removable from the surface of the aerofoil after the machining operation has been completed and the encapsulating alloy matrix removed. In this example the NICRO- BRAZE cement is dissolved off with dichlorethylene.

Further the coating must be sufficiently stable to withstand at least the temperature of the molten encapsulating alloy without detriment to its properties.

The coating is applied in this example as a viscous paint and is quick-drying, although other methods of application may be used e.g., dipping into a slurry comprising a mixture of cement and grit. The coating must obviously be sufficiently viscous not to run ofi the aerofoil before drying.

The encapsulating material is a low melting point alloy sold under the trade name CERROTRUE" which melts at 180 C. However other encapsulating materials are well known and may be used provided the workpiece is rigidly supported throughout the machining operation.

The above described coating on the surface of the workpiece rovides a k e ed surface for better adhesion of the encaps atrng maten By providing that the coating is readily removable from the surface of the workpiece the machining operation can be performed and both the coating and the encapsulating material removed without physical or chemical damage to the workpiece surface.

The invention has been described with reference to NICRO- BRAZE cement as the binder but it should be understood that alternative forms of binder may be used for example polyvinyl alcohol, cellulose based materials, poly-vinyl butyral, poly-methyl methacrylate. Poly-vinyl alcohol has the advantage that it can be dissolved in water.

We claim:

1. A process for producing a workpiece mounting for holding the workpiece during machining operations upon the workpiece, the process comprising the sequential steps of a. applying a settable coating, which comprises a grit in a binder, to a portion of the workpiece to provide a key,

b. casting a block of material around said coating when set, said cast material being solid at room temperature and said coating remaining stable at least up to the melting temperature of said cast material,

c. holding the workpiece by rigidly clamping the block of material,

d. performing the machining operations upon the workpiece,

e. releasing the workpiece,

f. removing the block of cast material,

g. removing the coating by dissolving the binder in a solvent which is non-corrosive to the workpiece material to yield the workpiece in a finished machined condition.

2. A process according to claim 1 in which the particle size of the grit is in the range between 100 and 200 British Standard mesh.

3. A process according to claim 1 in which the particle size of the grit is 180 British Standard mesh.

4. A process according to claim 1 in which the binder is a methyl cellulose based material in a solvent.

5. A process according to claim 1 in which the binder is poly-vinyl alcohol.

6. A process according to claim 1 in which the grit is aluminium oxide or silicon oxide.

7. A process according to claim 1 in which the workpiece is a gas turbine engine blade having an aerofoil portion to which the settable coating is applied. 

2. A process according to claim 1 in which the particle size of the grit is in the range between 100 and 200 British Standard mesh.
 3. A process according to claim 1 in which the particle size of the grit is 180 British Standard mesh.
 4. A process according to claim 1 in which the binder is a methyl cellulose based material in a solvent.
 5. A process according to claim 1 in which the binder is poly-vinyl alcohol.
 6. A process according to claim 1 in which the grit is aluminium oxide or silicon oxide.
 7. A process according to claim 1 in which the workpiece is a gas turbine engine blade having an aerofoil portion to which the settable coating is applied. 